This invention relates to a process for the preparation of toluenediamine (TDA) and its separation into desired and undesired components. In this process, dinitrotoluene is hydrogenated and any solvent present, as well as water and other by-products, are separated from the resulting reaction mixture by distillation. A separating wall column is used for the subsequent separation of the dewatered mixture into desired isomers and undesired isomers, by-products that are still present, and high-boiling compounds.
According to the state of the art, m-TDA (a 2,4- and 2,6-isomer mixture) is prepared by the hydrogenation of dinitrotoluene for use in the preparation of TDI by phosgenation. The hydrogenation can take place in the presence of solvents. The catalyst is conventionally separated off first. This can be carried out, e.g., by filtration or sedimentation. In addition to the TDA target product and water by-product, organic by-products are also formed in the hydrogenation. These include low boilers and high boilers. Low boilers are compounds whose boiling point is below that of TDA. High boilers are compounds having a higher boiling point than the TDA target product.
These by-products can severely interfere with the use of the TDA target product in applications, especially the preparation of toluene diisocyanate (TDI by phosgenation. It is therefore necessary to separate the reaction mixture obtained into its components.
If solvent is added in the hydrogenation reaction, it is separated off first. This is generally effected in known manner by distillation in a continuous distillation column, the solvent being recovered by a suitable procedure in a sufficiently pure state to be able to be used directly in the process without further purification. It is also possible to separate the solvent from the reaction product by distillation, together with some or all of the water formed, and then to recover the solvent in the required purity in another process stage. If the reaction is carried out without the addition of solvent, this solvent separation is of course unnecessary.
This is then conventionally followed by drying of the TDA, i.e. the removal of the water of reaction, which makes up approx. 40 wt. % of the reaction mixture obtained. In principle, this can be done by simply stripping the water off by heating the TDA solution under vacuum and driving the vapors off. However, in this simple procedure, the water separated off is not obtained in the purity required for problem-free disposal, but is always contaminated with TDA. The water of reaction is therefore better removed by distillation in a suitable distillation apparatus. This is done, e.g., by heating the crude TDA solution in a column to a temperature of 200° C., the water being obtained at the top in pure form if the column is operated at normal pressure or a slight excess pressure and has about 20 to 30 trays. The TDA is then withdrawn from the bottom and the last traces of water are removed by expansion into a vacuum of 30 to 300 mbar.
A variant of this process is described in EP-A-0 236 839. This disclosed process allows water and highly volatile compounds, and to some extent also low boilers, to be separated off.
The mixture obtained (crude TDA) conventionally is generally composed of m-TDA containing less than 10 wt. % of ortho isomers, less than 5 wt. % of high boilers, less than 5 wt. % of low boilers and less than 5 wt. % of water. Separation of the 2,3- and 3,4-isomers from the TDA isomer mixture is known. Thus, for example, U.S. Pat. No. 3,420,752 discloses the use of a distillation column for this separation, o-TDA being obtained at the top and m-TDA at the bottom. As the crude TDA obtained from the reaction normally contains low boilers and high boilers, it is immediately clear that, without further measures, the top product contains not only o-TDA but also low boilers. Likewise, the bottom product will contain not only m-TDA but also high boilers. In the case of m-TDA, these impurities constitute a disadvantage of this process because these high boilers are undesired in the target product, so they have to be separated off before further use of the m-TDA for example, in the preparation of TDI by phosgenation.
U.S. Pat. No. 6,547,933 describes a process in which m-TDA is obtained free of high boilers. In one embodiment of this process, the crude TDA is separated into its isomers in a distillation column. A partial stream is withdrawn as vapor from the bottom third of the distillation column and transferred to a condenser to give m-TDA free of high boilers. However, an appreciable part of the product stream is still made up of m-TDA containing high boilers, which are then used, e.g., in a downstream phosgenation. In another variant of the process, an m-TDA stream is withdrawn from the column and partially evaporated. The resulting vapor stream is condensed to give m-TDA free of high boilers. The unevaporated portion, which contains high boilers, is recycled into the column. Another m-TDA stream, again containing high boilers, is withdrawn from the bottom of the column. In both variants, the stream containing high boilers that is discharged from the bottom will contain a substantial part of the m-TDA produced. Thus the process can only be employed economically if this stream containing high boilers is utilized. The object of the present invention is therefore not achieved in this disclosed process.
The process disclosed in U.S. Pat. No. 6,359,177 achieves a complete separation of the high boilers from the m-TDA. This is done by first separating the TDA into its isomers in a distillation column. The mixture of m-TDA and high boilers obtained as the bottom product is separated into an m-TDA stream and a high boiler stream in a second apparatus, which is made up of an evaporator and a condenser. The m-TDA still contained in the high boiler stream is depleted in another stripping column and partially replaced with o-TDA. This gives a stream containing essentially high boilers and o-TDA, which is discharged and, e.g., incinerated. A second stream, composed of o-TDA and m-TDA, is recycled into the isomer distillation column. Variants of this process are also described in U.S. Pat. No. 6,359,177. This process achieves the object of minimizing the m-TDA losses in the high boiler stream to be discharged, but it entails increased equipment and energy costs.
In thermal separation technology, it is often desired to separate a multi-component mixture into its individual components. In the case of one inflow and two product streams, it is possible to use the top and bottom outflows of a distillation column. In the case of multi-component mixtures, a further split can be achieved by repeating the separation into two streams. The disadvantage is that this procedure requires additional equipment such as columns, condensers or evaporators. This in turn increases the operating energy requirements and the associated costs. Numerous publications address the problem of reducing the equipment and energy costs involved in separating a mixture of substances, the yardstick for the energy efficiency of a separation sequence being the Petlyuk system (cf., for example, R. Agrawal, Z. Fidowski, “Are thermally coupled distillation columns always thermodynamically more efficient for ternary distillations?”, Ind. Eng. Chem. Res., 1998, 37, pp. 3444-3454). In this configuration, a preliminary separation column separates the inflow into two streams using the split vapor stream of the stripping section of the main column and the split liquid outflow of the rectifying section of the main column. The resulting vapor and liquid streams leaving the preliminary separation column are enriched in low or high boilers. These two streams are introduced into the main column. This configuration offers advantages in terms of the purity of the product withdrawn as a side stream. On the other hand, the purity of the inflows into the stripping and rectifying sections of the main column is improved by this arrangement. A high purity of the three product streams is achieved in this way.
U.S. Pat. No. 2,471,134 discloses an improvement to this procedure in which the preliminary separation column and main column are combined in one apparatus that is divided in the middle by a separating wall. This column is equipped with an evaporator and a condenser. The column is then made up of 4 segments. These are a common rectifying section at the top of the column, a common stripping section at the bottom of the column, and a preliminary separation segment and a main segment that are located next to one another in the middle section of the column and are separated by a wall. The mixture is introduced onto or into the preliminary separation segment, the top product is drawn off above the common rectifying section and the bottom product is drawn off below the common stripping section. The intermediate-boiling product is withdrawn from the main segment as a side stream. This separating wall column offers advantages in terms of the hydraulics of the whole system and reduces the equipment costs of the Petlyuk process.